Certain optical devices, such as near-field scanning optical microscopes (“NSOMs”) and optical storage devices, may operate by scanning an optical probe (“probe”) over a sample. Depending on the mode of operation of the optical device, the probe may illuminate or collect electromagnetic (EM) radiation, or both. In these applications, optical probes include light guides with a coating that prevents EM radiation leakage, except at an aperture that is smaller than a wavelength of EM radiation.
In an NSOM, the probe and/or sample move such that the aperture passes over the area to be imaged; an image is constructed on a line-by-line or point-by-point basis. Accordingly, the spatial resolution achievable by an NSOM is not limited by the wavelength of the EM radiation, as in standard microscopy, but rather by the dimension of the aperture through which the EM radiation passes (i.e., a smaller aperture produces a higher resolution image). However, the transmission of EM radiation through a subwavelength aperture decreases significantly with aperture size; this limits the scanning rate and thus the rate at which the NSOM generates the image.
NSOMs may use several types of probes. One example of an NSOM probe includes an optical fiber wit a fiber core, cladding and a fiber end tapered to a diameter of about 100 nm. The sides of the fiber end are coated with metal; an end face of the fiber core is uncoated. An NSOM inputs EM radiation into the fiber, for example through an opposite (untapered) end of the fiber. In the tapered fiber end, the EM radiation is no longer contained within the fiber core by total internal reflection. Accordingly, much of the input EM radiation leaks out of the tapered sides of the fiber end, and the metal absorbs it. Only a small fraction of the input EM radiation thus transmits through the end face as output EM radiation. A ratio of output EM radiation to input EM radiation (a transmission efficiency) of 10–6 to 10–5 is typical for such a probe. The damage threshold of the metal coating operates to limit the intensity of EM radiation that may be supplied to the probe; typically, only a few nanowatts of power is transmitted to a sample.
Other NSOM probes, for example employing (non-fiber) light guides, are subject to similar tradeoffs between usable intensity and the damage threshold.
Interactions between photons and surface plasmons in patterned metal films can mitigate certain transmission limitations of tapered optical fibers. Lezec et al. (Science 297, 820 (2002)) shows, for example, that transmission through a sub-wavelength aperture in a metal film can be enhanced by several orders of magnitude if a bulls eye grating (or ruled) pattern of several microns diameter is fabricated in the metal surface surrounding the aperture. Placing this structure on the end face of a partially tapered optical fiber, or other NSOM probe, thus provides an NSOM probe with higher throughput.
Because an NSOM operates in the near field, the probe-to-sample distance is carefully controlled. The probe-to-sample distance is generally obtained by dithering the probe parallel to a sample surface and measuring an oscillation amplitude. A shear-force interaction damps the oscillation amplitude when the probe is within about 30 nm of the surface. Acceptable spatial resolution of the probe-to-sample distance by an NSOM is on the order of the width of the probe. For example, an NSOM probe with a 100 nm wide end may be used over surfaces with feature sizes on the order of 100 nm. But much smoother surfaces are required for the successful use of existing probes with bulls-eye plasmon structures, due to their much larger lateral tip dimensions (on the order of 5 microns).